1. Field of the Invention
The present invention relates to a new magnetic pole, a magnet apparatus using it, and a magnetic resonance imaging apparatus.
2. Description of the Prior Art
In recent years, in the field of a nuclear magnetic resonance imaging (MRI) apparatus, an MRI apparatus using the so-called open type magnet that static magnetic field generation sources are arranged opposite to each other vertically or horizontally across a scanning space is under-development. Such an MRI apparatus has a sufficient open character, permits the so-called IVR (interventional radiology), and greatly expands medical possibilities.
It is essential for a magnet for an MRI apparatus to produce a uniform static magnetic field of several ppm in a scanning space. The method for making the magnetic field of the scanning space uniform is broadly divided into two ways such as a method for using a plurality of coils and optimizing the arrangement thereof and a method for using the so-called magnetic pole and optimizing the surface shape thereof.
As a superconductive magnet suitable for an open-type MRI apparatus using the former method, the magnet apparatus disclosed in W099/27851 MAGNET APPARATUS AND MRI APPARATUS may be cited. The magnet is composed of two sets of magnet assemblies opposite to each other across the scanning area of an MRI image. Each magnet assembly has a plurality of superconductive coils, which are arranged almost symmetrically about the central axis. The main coils are composed of coils of positive polarity and negative polarity which are alternately arranged, thus the compact magnets can generate a highly uniform magnetic field.
On the other hand, a magnet apparatus using the latter method is disclosed in Japanese Application Patent Laid-Open Publication No. 8-172010 and U.S. Pat. No. 5,592,089. A typical example thereof is shown in FIG. 7, FIG. 5 is a vertical cross sectional view of a magnet. Support-shaped magnetic substances 5 and 6 and tabletop-shaped magnetic substrates 4a and 4b constitute a magnetic circuit, which lets magnetic flux generated by permanent magnets 3a and 3b pass through. Pole pieces 1a and 1b lead the magnetic flux generated by the permanent magnets 3a and 3b to the center of the magnet apparatus. Pole faces 2a and 2b, in order to make the magnetic flux distribution in the center of the magnet apparatus uniform, are not in a plane shape. Further, U.S. Pat. No. 5,592,089 indicates a structure of a magnetic pole that a circular ring-shaped hollow is formed between a cylindrical member and a ring-shaped member.
Japanese Application Patent Laid-Open Publication No 4-246330 indicates that to reduce changes in a magnetic field due to heat generation of a gradient coil, a heat insulating material is provided between the coil and a pole piece and Japanese Application Patent Laid-Open Publication No. 2-117106 and Japanese Application Patent Laid-Open Publication No 10-146326 indicate that to make a magnetic field in a measuring space uniform, a magnet or a coil for magnetic field adjustment is arranged in the outer periphery of a magnetic pole.
As a method for using a magnetic pole to make the magnetic field of the scanning space uniform, the former patent application indicates that irregularities are formed on the magnetic pole on the scanning space side, thus the flow of the magnetic field is controlled and the magnetic field is made uniform. However, in the magnet of this concept, a position error of the magnetic pole surface greatly changes the uniformity of the magnetic field, so that very severe production accuracy is required for the magnetic pole surface and after all, a problem arises that the cost of the magnet is increased. Further, a position error caused by the production accuracy of the magnetic pole surface cannot be avoided, so that when many magnets are manufactured, the uniformity of the magnetic field in the scanning space inevitably varies greatly between the respective magnets, thus shimming is difficult, and it is difficult to economically manufacture many magnets for generating a magnetic field of several ppm in the scanning space.
Further, in the circular ring-shaped hollow indicated in U.S. Pat. No. 5,592,089, no uniform magnetic field can be formed.